This invention relates to wireless communication systems, and in particular to Wireless Personal Area Network (WPAN) and BLUETOOTH communication systems.
BLUETOOTH communication systems provide pervasive connectivity, especially between portable devices like mobile phones, laptop computers, personal digital assistants (PDAs), and other nomadic devices. A BLUETOOTH system, or piconet, typically uses frequency hopping (FH), with an air interface between devices that is organized as a succession of time slots that can include data packets. As long as the system devices hop in synchrony and with the same phase, they simultaneously use the same channel and thus stay in contact with each other. BLUETOOTH communication systems are currently specified by Version 4.0 of the Bluetooth Specification (30 Jun. 2010), which is promulgated by the Bluetooth Special Interest Group and can be found at www.bluetooth.com.
Devices operating in a low-energy mode defined by the Specification use frequency division multiple access (FDMA) and time division multiple access (TDMA), with three of forty FDMA physical channels allocated as advertising channels. A TDMA-based polling scheme is used in which one device transmits a data packet at a predetermined time and a corresponding device responds with a packet after a predetermined interval. Devices that transmit advertising packets on the advertising channels can be called Advertisers, and devices that receive advertising on the advertising channels without intending to connect to the Advertiser can be called Scanners. At the start of an advertising event, the Advertiser transmits an advertising packet corresponding to an advertising event type. Depending on the type, a Scanner can transmit a request to the Advertiser on the same advertising channel, and the Advertiser can reply with a response on the same advertising channel. Advertising is described in, for example, Volume 1, Part A, Subsection 4.2.2.2 of the Bluetooth Specification.
An active scanning procedure includes as well a point-to-point interaction between an Advertiser and Scanners in the form of Scan Request packets transmitted by the Scanners to the Advertiser and Scan Response packets transmitted by the Advertiser to the Scanners. Active scanning and other aspects of the advertising state are described in Volume 6, Parts B and D, of the Bluetooth Specification. Part B is a Link Layer Specification for the low energy mode of operation.
Active scanning can be simultaneously executed by two (or more) Scanners with the same scanning settings, and so a backoff procedure that minimizes collisions between Scan Requests is required by Subsections 4.4.3 and 4.4.3.2 of the Link Layer Specification. The backoff procedure is used at the Scanner side when missing Scan Responses from an Advertiser reveal that collisions have occurred between Scan Requests of two or more Scanners. The backoff procedure uses two parameters, backoffCount and upperLimit, to restrict the number of colliding Scan Request packets. After success or failure of receiving a Scan Response to a Scan Request, the backoffCount parameter is set to a new random integer between one and the upperLimit parameter. Scan Requests can be sent only when the backoffCount parameter becomes zero, which can cause problems for some Scanners.
Some approaches to problems in signal collision avoidance are known, but those approaches do not solve the problems in the backoff procedure described above, such as prevention of bidirectional communication by a randomly determined Scanner.
For example, U.S. Patent Application Publication No. US 2009/0257396 A1 by Eliezer et al. for “System and Method of Adaptive Frequency Hopping with Look Ahead Interference Prediction” discloses dynamic adaptation of frequency hopping sequences in a BLUETOOTH system so that frequencies that regularly experience collisions during a specific time slot are replaced. A mechanism for predicting and avoiding collisions between a BLUETOOTH device and non-BLUETOOTH systems using a fixed frequency range within the BLUETOOTH frequency range are described.
European Publication EP 1233583 B1 by Schmidt et al. for “Method and Arrangement for Providing a Response to an Inquiry in a Wireless Bluetooth System” describes inquiry messaging, and an improvement to the behavior of slave devices in BLUETOOTH inquiry scenarios, so as to minimize the inquiry time that a master device spends in detecting slave devices.